The present invention relates to apparatus for the manufacture of plastic bags from a tubular web of film which is advanced intermittently in the forward direction. The apparatus has cyclically operating sealing and cutting devices for producing a bottom-weld seam of each bag (such seams extending transversely of the film web) and for separating the bags from the film web. The apparatus also has a stacking station arranged behind the cutting device as seen in the direction of advance, and the stacking station is equipped with a plurality of movable needles which can be moved by a suitable drive, between (a) a needling position which is adjacent the cutting device (in which needling position the open-end region of bags separated by the cutting device can be pushed onto the needles by a movable hold-down plate, in order to form a stack of bags which rests on a slotted resting surface), and (b) a transfer position which is further away from the cutting device in the direction of advance (in which transfer position the needles are withdrawn from the stack of bags, thereby transferring the latter to a conveyor device, for removal of successive stacks in the direction of advance).
One apparatus of this kind is already known from West German Unexamined Patent Application (Offenlegungsschrift) No. 2,833,236. In this known apparatus, the needles are arrayed in two straight needle rows which are parallel to each other, each of them extending transverse to the direction of bag advance, and over at least a large part of the width of the bags to be needled. In this known apparatus, the needles of one row of needles are in staggered relation to the needles of the other row of needles, so that needles of the two rows pass through each other in the manner of a comb, i.e., they can move independently of each other. The rows of needles are so moved, each by its own drive (which is associated with each row of needles and can be controlled independently from the action of the other drive), that during transfer motion of one row of needles from its needling position, the other row of needles (which had remained in engagement with the stack which was previously transferred to the conveyor device) is disengaged from the stack and moved into the needling position; transfer motion of the one row of needles is stopped as soon as the stack needled on it comes to lie on the conveyor device, but the needles of said one row remain temporarily engaged to the stack.
In the known apparatus, the presence of two independently movable rows of needles is deemed necessary for positive control of bag stacks, in that the stack of bags last fed to the conveyor device can be held secure by continuous engagement of the needle points of one row until needles of the other row have accumulated a stack and have begun to move out of needling position, or until at least one or more newly produced needled bags protectively cover the front edge of the last stack fed to the conveyor device. Such securing of the last stack fed to the conveyor device is intended to prevent the last stack from disarray, in the presence of a stream of film-tensing blast air from nozzles disposed upstream of the sealing and cutting devices. The advantage of positively securing of the stack last fed to the conveyor device is, however, obtained in the known apparatus at substantial cost disadvantage, caused by the presence of two rows of needles which must be moved independently of each other. Since separate drives are needed for the two rows of needles, there is structural complexity, resulting in correspondingly high manufacturing cost. Furthermore, there is the disadvantage of a high consumption of energy caused by the fact that in each case the masses of two drive systems must be accelerated and decelerated for intermittent displacement of the two rows of needles.